Semi-solid Metal Processing
The present invention is compatible with semi-solid metal processing, also known as semi-solid metal casting. Semi-solid metal casting (SSM), also known as thixocasting, rheocasting, thixoforming or thixomolding, is a near net shape process in the production of parts out of non-ferrous metals, such as aluminium, copper, or magnesium. The process combines the advantages of casting and forging. The process is named after the fluid property thixotropy, Which is the phenomenon that allows this process to work. Simply, thixotropic fluids shear when the material flows, but thicken when standing. The potential for this type of process was first recognized in the early 1970s. See, e.g., Young, Kenneth P., Semi-Solid Metal Casting: Reducing the Cost of Copper Alloy Parts, Massachusetts Office of Technical Assistance, http://web.archive.org/web/20061007163908/http://www.mass.gov/envir/ota/publications/pdf/semi solid metal fact sheet.pdf; Lowe, Anthony; Ridgway, Keith; Atkinson, Helen (September 1999), “Thixoforming”, Materials World 7 (9): 541-543, http://www.azom.com/details.asp?ArticleID=1373; each of the preceding is incorporated by reference herein.
SSM is typically done at a temperature that puts the metal between its liquidus and solidus temperature. Generally the metal is 30 to 65% solid. The metal can have a low viscosity, and to reach this low viscosity the material can have a globular primary surrounded by the liquid phase. The temperature range possible depends on the material and for aluminum alloys is 5-10° C., but for narrow melting range copper alloys it can be only several tenths of a degree.
Semi-solid casting is typically used for high-end castings. For aluminum alloys typical parts include engine suspension mounts, air manifold sensor harness, engine blocks and oil pump filter housing. Kapranos, Proc. 10th Inter. Conf. Semi-Solid Processing of Alloys and Composites, Aachen, Germany & Liege, Belgium, 2008. For magnesium alloys, semi-solid casting is typically used to produce extremely thin walled castings, such as computer and camera bodies.
There are a number of different techniques to produce semi-solid castings. For aluminum alloys some common processes include thixocasting and rheocasting. Stephen P. Midson, Rheocasting Processes for Semi-Solid Casting of Aluminum Alloys, Die Casting Engineer, January 2006, incorporated herein by reference. Other process such as strain induced melt activation (SIMA) and RAP can also be used with aluminum alloys, although are less common commercially. With magnesium alloys, a common process is thixomolding. S. LeBeau & R Decker, “Microstructural Design of Thixomolded Magnesium Alloys”, Proc. 5th Inter. Conf. Semi-Solid Processing of Alloys and Composites, Golden, Colo., 1998, incorporated herein by reference.
Thixocasting utilizes a pre-cast billet with a non-dendritic microstructure that is normally produced by vigorously stirring the melt as the bar is being cast. Induction heating is normally used to re-heat the billets to the semi-solid temperature range, and die casting machines are used to inject the semi-solid material into hardened steels dies. Thixocasting is being performed commercially in North America, Europe and Asia. Thixocasting has the ability to produce extremely high quality components due to the product consistency that results from using pre-cast billet that is manufactured under the same ideal continuous processing conditions that are employed to make forging or rolling stock. Stephen P. Midson, Semi-Solid Casting of Aluminum Alloys: An Update, Die Casting Engineer, September 2008, incorporated herein by reference.
Rheocasting develops the semi-solid slurry directly from the liquid, normally adjacent to the die casting machine. There are a large number of rheocasting processes that have been proposed over the past 10 years or so, and they generally differ in the method used to generate the semi-solid slurry. 18 different rheocasting techniques were documented in a recent publication.
For magnesium alloys, thixomolding uses a machine similar to injection molding. In a single step process, room temperature magnesium alloy chips are fed into the back end of a heated barrel through a volumetric feeder. The barrel can be maintained under an argon atmosphere to prevent oxidation of the magnesium chips. A screw feeder located inside the barrel feeds the magnesium chips forward as they are heated into the semi-solid temperature range. The screw rotation provides the necessary shearing force to generate a globular structure suitable for semi-solid casting. Once enough slurry has accumulated, the screw moves forward to inject the slurry into a steel die. Stephen P. Midson, Robert K. Kilbert, Stephen E. Le Beau & Raymond Decker, “Guidelines for Producing Magnesium Thixomolded Semi-Solid Components used in Structural Applications”, Proc. 8th Inter. Conf. Semi-Solid Processing of Alloys and Composites, Limasol, Cyprus, 2004, incorporated herein by reference.
In the SIMA method the material is first heated to the SMM temperature. As it nears the solidus temperature the grains recrystallize to form a fine grain structure. After the solidus temperature is passed the grain boundaries melt to form the SSM microstructure. For this method to work the material should be extruded or cold rolled in the half-hard tempered state. This method is generally limited in size to bar diameters smaller than 37 mm (1.5 in); because of this only smaller parts can be cast.
Reactive Munitions
The present inventions can be used to benefit reactive munitions. Generally, there is a desire to provide munitions with high enthalpic energy release and controlled fragmentation and breakup. There is also a desire to provide for high velocity fragment dispersion. The case must still provide the structural properties necessary to withstand the propulsion events. Previously, the case was required to be formed of a high strength material, for example steel, and reactive materials were housed within or coated onto the case. As examples, see U.S. patent publications 20040016355; 20040112241; 20060011086; 20060288897; 20080229963; each of which is incorporated herein by reference. The present inventions can provide more efficient incorporation of reactive materials in munitions. The present inventions can also provide for higher velocity fragment dispersal than was previously attainable.
Other Applications
There are several other applications for the present inventions. For example, embodiments of the present invention can be used in structural engineering applications where the thermal properties of aluminum are desired, but the mass properties of steel (e.g., density) are desired. Also, embodiments of the present inventions can be applied in sporting goods, as the composite materials enabled by the present inventions can have unique properties that can be desirable for sporting equipment such as some types of golf clubs.